As the most frequently diagnosed cancer in North America, basal cell carcinoma (BCC) affects nearly a million new patients each year. At present, the exact cells that give rise to BCCin other words, the cells-of-origin for these tumorsare currently unclear, as multiple studies have reported conflicting results. During the K99 phase of this grant, I identified wounding as a modulator of tumor cell-of-origin. In the absence of wounding, hair follicle stem cells in the bulge region are incapable of forming tumors induced by an oncogene that transduces high level Hedgehog (Hh) signaling. However, upon wounding these same stem cells leave the bulge and transit to the site of injury, where they gain the ability to form tumors. Building upon this previous work, this grant seeks to examine in greater detail the possible cells-of-origin for BCC, as well as the signaling pathways that impinge upon tumor formation. The Aims of this proposal will include determining whether 1) Wnt, another important cell signaling pathway, collaborates with Hh in modulating BCC formation; 2) whether stem cells located in the hair follicle isthmus are capable of developing tumors; and 3) whether an important receptor previously implicated in cell migration, CXCR4, plays a role in wound-induced BCC tumorigenesis. The sum of these studies will likely expand our knowledge of BCC, and hopefully increase our understanding of other tumors that rely upon deregulated Hh signaling.